Conventionally, there is a communication method called “MIMO (Multiple-Input Multiple-Output)” whereby a plurality of sequences of transmission data are modulated, modulated data are transmitted simultaneously from a plurality of antennas and the data communication speed is thereby enhanced. The receiving side receives transmission signals from the plurality of antennas using a plurality of antennas.
Here, the received signal obtained at each receive antenna consists of a plurality of modulated signals mixed together in the propagation space, and therefore reconstructing the data corresponding to each modulated signal requires a variation (hereinafter, referred to as a “channel fluctuation”) of each modulated signal in the propagation path to be estimated. Therefore, the transmission apparatus inserts known signals such as pilot symbols in the modulated signal beforehand and the reception apparatus estimates a channel fluctuation in the propagation space between each transmit antenna and each receive antenna based on the known signals inserted in the modulated signal. Each modulated signal is demodulated using this channel fluctuation estimated value.
One such method is a method whereby an inverse matrix calculation is carried out on a matrix whose elements consist of channel fluctuation estimated values to separate the signal into respective modulated signals. Another method is one whereby a candidate signal point positions are found using channel fluctuation estimated values and a maximum likelihood detection (MLD) is carried out between these candidate signal point positions and the received signal point position to thereby reconstruct the data transmitted with each modulated signal.
A communication technology using such multi-antennas is disclosed, for example, in Non-Patent Document 1. Hereinafter, the contents disclosed in this Non-Patent Document 1 will be explained briefly using FIG. 102. Multi-antenna transmission apparatus 1 inputs transmission signal A and transmission signal B to modulated signal generation section 3. Modulated signal generation section 3 applies digital modulation processing such as QPSK (Quadrature Phase Shift Keying) and 16QAM (Quadrature Amplitude Modulation) to transmission signals A, B and sends out baseband signals 4, 5 obtained in this way to radio section 6. Radio section 6 applies radio processing such as up-conversion and amplification to baseband signals 4, 5 and sends out modulated signals 7, 8 obtained in this way to antennas 9, 10. In this way, multi-antenna transmission apparatus 1 sends modulated signal 7 of transmission signal A from antenna 9 and modulated signal 8 of transmission signal B from antenna 10 simultaneously.
Multi-antenna reception apparatus 2 inputs received signal 12 received from antenna 11 to radio section 13 and also inputs received signal 16 received form antenna 15 to radio section 17. Radio sections 13, 17 apply radio processing such as down-conversion to received signals 12, 16 and send out baseband signals 14, 18 obtained in this way to demodulation section 19.
Demodulation section 19 obtains received digital signal 20 of transmission signal A and received digital signal 21 of transmission signal B by detecting baseband signals 14, 18. At this time, Non-Patent Document 1 describes a method of carrying out an inverse matrix calculation on a channel estimation matrix to obtain received digital signals 20, 21 and a method of carrying out a maximum likelihood detection (MLD) to obtain received digital signals 20, 21.
Furthermore, as a conventional transmission method using a plurality of antennas, a technology as disclosed in Non-Patent Document 2 for realizing high quality (with a good error rate characteristic) data transmission by transmitting time and space block codes (STBC: Space-Time Block Code) is known. Hereinafter, the contents disclosed in this Non-Patent Document 2 will be explained using the accompanying drawings.
As shown in FIG. 103, the transmission apparatus has a plurality of antennas AN1, AN2 and sends signals simultaneously from antennas AN1, AN2. The reception apparatus receives the plurality of signals sent simultaneously by antenna AN3.
FIG. 104 shows the frame configuration of signals transmitted from antennas AN1, AN2. Transmission signal A is transmitted from antenna AN1 and at the same time, transmission signal B is transmitted from antenna AN2, Transmission signal A and transmission signal B consist of symbol blocks made up of the same symbol arranged a plurality of times so as to obtain a coding gain and a diversity gain.
This will be explained in further detail. In FIG. 104, S1, S2 denote different symbols and “*” indicates a complex conjugate. In space-time block coding, at time i, symbol S1 is transmitted from first antenna AN1 and at the same time symbol S2 is transmitted from second antenna AN2 and at next time i+1, symbol −S2* is transmitted from first antenna AN1 and at the same time symbol S1* is transmitted from second antenna AN2.
Antenna AN3 of the reception apparatus receives a signal which is a combination of transmission signal A affected by transmission path variation h1(t) between antenna AN1 and antenna AN3 and transmission signal B affected by transmission path variation h2(t) between antenna AN2 and antenna AN3.
The reception apparatus estimates transmission path variations h1(t) and h2(t), separates original transmission signal A and transmission signal B from the combined received signal using the estimated values and then demodulates each symbol.
In this case, if signals subjected to space-time block coding as shown in FIG. 104 are used, it is possible to combine symbols S1, S2 at a maximum ratio irrespective of transmission path variations h1(t), h2(t) when the signal is separated, and therefore it is possible to obtain a large coding gain and diversity gain. As a result, the reception quality, that is, the error rate characteristic can be improved.    Non-patent Document 1: “Multiple-antenna diversity techniques for transmission over fading channels” IEEE WCNC 1999, pp. 1038-1042, September 1999.    Non-patent Document 2: “Space-Time Block Codes from Orthogonal Design” IEEE Transactions on Information Theory, pp. 1456-1467, vol. 45, no. 5, July 1999